Barrel Twist and the Greenhill formula

[ncsurveyor]

I have an unhealthy fascination with T/C Maxi Balls.

I spreadsheeted the Greenhill formula for barrel twist and came up with:

32 cal = 24 twist
45 cal = 40 twist
54 cal = 48 twist

(I haven't purchased maxi's in other calibers yet.)

I have a 45 cal in a 48 twist, but was planning on getting a 32 cal barrel and a 54 cal for future builds (underhammers) so I was curious if folks found out what twists worked - or didn't - with maxi's.

If you know the bore/groove, that info would help as well.

I could check other calibers as well, but don't have the length for 36, 40, 50 or 58 cal maxi's.

jc

 

[Zonie]

I don't see a listing for a Thompson Center Maxi-Ball smaller than .45 caliber in my 1996 TC catalog .

That said, the TC Hawken and Renegade both use a 1:48 twist in their .45, .50 and .54 caliber barrels.

TC designed these Maxi-Ball's for their rifles so I feel safe in saying their .45, .50 and .54 Maxi-Balls should shoot fine in a 1:48 twist barrel.

IMO, the Greenhill formula was designed to give rough estimates of the ball/bore/twist ratios so the answers it gives are not actually hard limits.

The only way to find out how some bullet will really behave in a barrel is to shoot some targets.

 

[Maven]

"I have a 45 cal in a 48 twist, but was planning on getting a 32 cal barrel and a 54 cal for future builds (underhammers) so I was curious if folks found out what twists worked - or didn't - with maxi's." ...ncs

I fired T/C Maxi-Balls in 1:48 twist (.45cal.) and 1:28 twist bbls. (.50cal. Knight Big Horn) and have had no problems with accuracy. The bigger question is the dimension of a given bore and how well the Maxi-Ball fits that bore. Since I cast my own MB's, I have experimented with both Lyman and T/C MB molds and find the latter was held to a tighter tolerance than the former, To wit, undersized Maxi-Balls shot inaccurately in either twist bbl., but those that engraved at the muzzle and required a bit more effort to start, were much more accurate. Btw, a Lee REAL bullet can be equally accurate IF it fits your bore. Lastly, you can enhance the accuracy of Maxi-s or REALs by using a cardboard or felt wad over the powder charge.

 

[ncsurveyor]

Zonie, you are correct about the 32. The mould I have for them is a Lee.

I guess calling them T/C maxi's is habit for me, like calling tissue paper Kleenex, I suppose.

As far as greenhill goes, I haven't noticed the correlation to twist as readily in muzzleloaders as I have had in centerfire rifles. But playing the numbers was interesting - especially when I started looking at round balls and twist rates.

Back to the maxi ball, like Maven says, fit may be more critical. My guess is we are looking to have groove diameter only slightly larger than the upper driving band of the maxi, with bore diameter slightly smaller or at the diameter of the bottom band, so it can start easier?

 

[mooman76]

They did have a T/C in 32 and 36 but are hard to come by as they were discontinued some time ago. I believe mountain Moulds makes a copy now though.

 

[don hepler]

I have two boxes of 36 cal. Maxi-Ball that I bought out of a close-out junk bin, when I was at the big machine-gun shoot down in Louisville Ky. I've been meaning to test them in my TC Seneca, but forget too.

 

[M. De Land]

I'm going by memory now as I don't have the information in front of me but one of our more astute members of the Black Powder Cartridge fraternity( Dan Theodore) who has since passed away, came up with using a factor of 125 instead of the recommended 150 multiplier in the formula.
Dan was an electrical engineer with a shooting addiction and ferocious experimenter.
In the formula C equals 150 and if the velocity is over 2800fps C should be raised to 180.
What Dan discovered is that C should be 125 for the true velocity factor of Black Powder cartridge bullets. Again, if I'm remembering correctly.
He found the lower multiplier in the formula to produce a more accurate indicator of lead bullet stability from barrel twist.
Dan was the inventor of the now famous "Money Bullet" which in one form or another is setting all the current national match records.

 

[ncsurveyor]

Thanks M.D.

Makes me curious if I could derive an incremental C value that is based on velocity.

 

[hawkeye2]

"using a factor of 125 instead of the recommended 150 multiplier in the formula."

M. D. wasn't there a thread on this subject on here quite a number of years back? I never had much luck with the search here so I won't be looking it's just that I seem to remember the subject.

 

[ncsurveyor]

Gentlemen, I found 219 posts mentioning Greenhill that are over 1 year old, and 11 within the last year.

Guess I should have searched before I asked. I may have to prep that for a rainy day.

 

[M. De Land]

I got this from the Shiloh forum I have been a member of since 2006. I don't remember the discussion on it here.

 

[nkbj]

Don't forget that the maxi isn't a typical bullet such as Greenhill should normally be used for, having substantial fletching to keep it straight.

 

[hawkeye2]

Perhaps I remember the discussion from another forum too though not Shiloh.

 

[M. De Land]

That's true but it should be close enough to come up with a practical stabilizing pitch for a maxi being a conical rather than a ball.
I made a Maxi mold for a .58 caliber rifle I own with a pitch of 1 in 60 and it shoots superbly to the 100yards I have tested it. It weighs 620 grains and the accuracy load is 150 grains of 2F Goex but it is an absolute bear for recoil.
I always use a shoulder pad and don't enjoy shooting that load a bit but it is a very accurate combination if you can eliminate a flinch.

 

[RedFeather]

As I understand it, the Green hill formula is not a one-size-fits-all. It varies based upon projectile speed. Years ago, I plugged it into an Excell spreadsheet and found that a constant of 125 gave generally accepted BP ROT's. I think "hot rock" varmint shooters go up the other way.